1. Field of the Invention
The present invention relates to a vitreous silica crucible suitable for manufacturing a silicon single crystal, and, more specifically, a vitreous silica crucible that can suppress deformation by heat.
2. Description of the Related Art
A semiconductor silicon single crystal is, in general, manufactured by the Czochralski method (CZ method) in which a single crystal is pulled from silicon melt obtained by heating and melting raw material polycrystalline silicon. A bowl-shaped vitreous silica crucible having an upper opening is used to retain the polycrystalline silicon. The vitreous silica crucible is manufactured by use of natural silica powder or synthetic silica powder as raw material.
Such a vitreous silica crucible, in general, includes plural layers with different properties (See, e.g., JP-A-2004-352580). The layers include a transparent vitreous silica layer containing scarce bubbles (hereinafter, referred to as “transparent layer”) on the inner surface, and a vitreous silica layer containing a large amount of bubbles (hereinafter, referred to as “bubble-containing layer”) on the outer surface.
The innermost layer contacting silicon melt in severe conditions, which are, for example, at 1400 degrees C. for scores of hours, is comprised of a smooth transparent layer having scarce bubbles. When bubbles exist, the bubbles can promote growth of a micro crystal such as cristobalite during melting silicon or pulling a single crystal, and in addition, when the inner surface of the crucible is corroded in the course of the pulling, the bubbles can expand and cause peeling of the micro crystal, which results in deterioration of the single crystallization yield. Therefore, the innermost layer is comprised of a bubble-free layer. However, in order to prevent melt surface vibration of silicon melt and maintain the crucible strength during pulling of a silicon ingot, the bubble content ratio of the transparent layer is set to be 0.1% or more in an upper portion of the straight body portion, which is a portion corresponding to the melt surface of the silicon melt.
On the other hand, the outermost surface of the vitreous silica crucible is comprised of a bubble-containing layer in order to (1) suppress heat release from silicon melt and enhance the heat keeping effect to improve the uniformity in the heated state and the heat efficiency, (2) dissipate heat from the surrounding of the vitreous silica crucible to prevent localized heating, and (3) maintain the crucible strength during pulling of a single crystal.
Nowadays, in pulling a silicon single crystal, the time to pull a silicon single crystal is increased three or more times compared with a conventional pulling process, to obtain a silicon single crystal having an excellent property. However, when the vitreous silica crucible is heated for a long time, bubbles contained in the vitreous silica crucible expand eminently, which lead to deformation or corrosion of the vitreous silica crucible. This causes problems such as that melt surface vibration excessively occurs and turbulent flow occurs locally. Furthermore, the deformation changes the crucible inner volume. The change of the inner volume leads to the change of the melt surface position of silicon melt. Furthermore, it can prevent single crystallization, which leads to a drop in the single crystallization yield and deterioration in quality.
For example, as shown in FIG. 13, the vitreous silica crucible C for pulling a single crystal includes an inner layer (transparent layer) C1 contacting silicon melt during pulling of a single crystal, and an outer layer (bubble-containing layer) C2. The bubble content ratio of the upper portion C3 of the crucible inner layer C1 is different from that of the lower portion C4.
The upper portion C3 of the crucible inner layer C1 is a region from the rim portion C5 at the upper end to the lowered melt surface position at the beginning stage of pulling of a silicon single crystal. In more detail, the upper portion C3 is a region from the rim portion C5 to the melt surface position lowered until a shoulder of a silicon single crystal is formed.
In pulling of a silicon single crystal by use of the CZ method, melt Y filled in a crucible C is heated by use of a heater H, and a seed crystal K is contacted with melt Y to grow a single crystal I. A shoulder is formed after necking to remove dislocation.
The lowered melt surface position is shown as reference symbol Y0 in FIG. 14, and is a height position at the end of forming the shoulder Is for enlarging a diameter to form a straight body portion to be sliced to produce wafers, and in other words, is a height position at the beginning of the formation of the straight body portion It. In FIG. 14, the state at the end of formation of the shoulder Is and the straight body portion It are shown in a solid line and a dotted line, respectively.
In pulling of a single crystal I, in particular, at the beginning, there is observed a phenomenon that the melt surface of silicon melt periodically vibrates. When the melt surface vibration occurs, there occur problems that a seed crystal cannot be connected to the melt surface and silicon is polycrystallized during pulling. The reason for the melt surface vibration is the generation of SiO gas and so on which is generated by the reaction between silicon melt and vitreous silica, and the reaction is activated by increase of the pulling temperature and decrease of the ambient pressure (See JP-A-2003-081689, JP-A-2002-326892, and JP-A-2002-154894). In particular, the pulling is unstable until a shoulder is formed after growth of a seed crystal connected to silicon melt, and thus can be easily affected by the melt surface vibration.
In order to prevent melt surface vibration caused by expansion of the incorporated bubbles, and deformation and corrosion of a vitreous silica crucible, there is known a crucible having an upper one third portion of the crucible whose OH group content in the inner layer is smaller by 100 ppm or more than that of the upper half of the outer layer and having a lower two third portion of the crucible whose OH group content in the inner layer is larger by 100 ppm or more than that of the lower half of the outer layer (See JP-A-2005-330157).
Furthermore, there is also known a vitreous silica crucible having a transparent layer made of transparent vitreous silica on the inner surface side and a opaque silica layer made of opaque vitreous silica having many closed bubbles on the outer surface side, wherein the red fluorescence intensity of the transparent layer is more than 0.05 and 0.5 or less, and the difference of the average value of the red fluorescence intensity between the bubble-containing layer and the transparent layer is 0.9 or more and the ratio is 3.0 or more (See JP-A-2006-089301).